Hi,
i'm not an injection expert, but since you've done a lot of the sensor work involved with the MPi engine and since we're talking about a siamese ports head i'd look into this:
The inner two plugs richer than outer two.
Due to the siamese ports, shared exhaust and camshaft desing the two inner cylinders will burn different than the two outer.
With 'standard' timing you'll have the outer richer. From memory the lambda reads the inner cylinders on a standard MPI (correct me if i'm wrong) which are those you have rich..so you have either something wrong with the centre port or the cam is slightly out of timing.
Now i'd look into two things. What exhaust manifold are you running and is it leaking? Can you test the lambda sensor?
Also i see you changed coil and leads, but what about a fresh set of NGK R plugs?
